Foam sheets using petroleum resins such as polyvinyl chloride (PVC) and the like are widely used to various building structures such as houses, mansions, apartments, offices, shops, and the like.
Such foam sheets are prepared through extrusion or calendering of a resin such as polyvinyl chloride (PVC), and the like. However, since raw materials for these sheets are obtained from limited resources such as crude oil, it is anticipated that the depletion of petroleum resources will lead to various problems related to the supply of raw materials.
Moreover, considering increasing interest in environmental problems in recent years, there is a problem in that polyvinyl chloride (PVC) foam sheets are likely to emit toxic substances and create an environment burden when discarded.
To resolve such problems, a polylactic acid (or polylactide) resin, which is extracted and synthesized from plant resources, is spotlighted as a material capable of replacing petroleum resins in recent years. Polylactic acid is prepared by polymerization of lactic acid, which can be obtained by fermentation of starch extracted from renewable plant resources (corn, potatoes, sweet potatoes, and the like), and is an environmentally friendly resin enabling reduction in CO2 emissions and conservation of non-renewable energy sources. A number of references including Korean Patent Publication No. 10-2008-0067424 disclose foam sheets using a polylactic acid resin.
However, since polylactic acid is easily hydrolyzed at certain humidity and temperature, there is a drawback in that a board prepared from the polylactic acid resin clings to a processing apparatus upon thermal lamination or is not easily stacked in multiple layers due to lack of viscoelasticity upon high temperature processing, as compared with existing boards prepared from a PVC resin. Therefore, it is an important issue to improve water resistance and processability of the foam sheet prepared from the polylactic acid resin.
Further, since polylactic acid has high crystallinity and a relatively low molecular weight and exhibits low melt strength, polylactic acid has a problem of low expansion ratio upon foaming.